Why does mercutio engage in a duel with tybalt instead of letting romeo take care of the challenge?

Mercutio fights Tybalt because Romeo refuses to. When Tybalt approaches Romeo and demands that he fight, Romeo responds by  telling Tybalt "I never injured thee, But love thee better than thou canst devise." He is saying that he has a reason, which he cannot say, to love Tybalt.. Mercutio is appalled by Romeo's behavior and cannot stand by and watch his friend be disrespected. He says, "O calm dishonourable, vile submission!" In this line, he is calling Romeo's submissive behavior dishonorable." He then takes matters into his own hands and draws on Tybalt.